The present invention relates to mobile drilling rigs and, in particular, to an arrangement of a motor, an air compressor and hydraulic pumps on a mobile drilling rig.
A conventional mobile, steerable drilling rig 10, depicted in FIGS. 1 and 2, compresses a platform 12 below which a drivable ground support is attached, such as a pair of rotatable wheels (not shown) or two rotatable endless carrier tracks 14, disposed on respective sides of the platform. An operator's cab 16 is disposed at a rear end of the platform. Situated on the platform in front of the cab 16 is a drilling assembly 17 for drilling holes downwardly into the ground. The drilling assembly 17 includes a swingable beam 18, such as a mast whose lower end is pivotably connected to a fixed pedestal 18a to be swingable about a horizontal axis extending perpendicularly to a front-to-rear extending longitudinal axis A of the rig. Thus, the mast 18 can be swung by hydraulic devices 21 from the vertical state depicted in FIG. 1 to a horizontal state (not shown) in which the free end of the mast sits on a mast rest 20 disposed at a front end of the platform.
The mast is oriented horizontally when tramming, i.e., when driving the rig from one site to another. On the other hand, the mast is oriented vertically during a drilling operation. Mounted on the mast is a hydraulically powered raising/lowering mechanism for raising or lowering a drill string having a drill bit at its lower end. The drill string is comprised of series of interconnected drill rods that are stored in a carousel mounted on the mast. Hydraulic devices such as motors or cylinders are provided for rotating the carousel.
Hydraulic pumps 27 are provided (FIG. 3) for providing pressurized fluid to the various hydraulic devices and hydraulic motors.
In order to flush cuttings from a hole as it is being drilled, it is common to direct compressed air downwardly through the drill string to the front face of the drill bit. The cuttings become entrained in the airflow and are brought to the surface as the air travels upwardly along the annulus surrounding the exterior of the drill string. The compressed air also serves to cool the cutting elements of the drill bit. The compressed air is produced by a compressor, typically a screw compressor 22. The compressor delivers compressed air to an air reservoir 19 (see FIG. 4) from which it is conducted to the drill string. Lubricating oil is mixed with the compressed air for lubricating the compressor. The lubricating oil is separated from the compressed air within the reservoir 19 and is conducted back to the compressor and the compressor gear box 22a through a conduit 19a, as depicted in FIG. 4. The oil is propelled through the conduit 19a by a pressure difference between the reservoir and the gearbox. The oil is then sucked out of the gear box 22a and into the compressor inlet through a conduit 19b. 
In order to drive the screw compressor 22 and the hydraulic pumps 27, it is conventional to employ a motor 26, such as a fuel-driven engine (e.g., a diesel engine) or an electric motor for example. In a typical arrangement, the compressor 22 the motor 26, the motor drive shafts, the pumps 27, and gearboxes for the compressor and the pumps are laid out in a line extending parallel to the longitudinal axis of the rig, as shown in FIGS. 2 and 3.
The gearing 23 on the compressor is disposed in a gearbox situated between the rear fly wheel end of the motor and the compressor, as shown in FIG. 3. A main gear 23a of the gearing is driven by the motor and it, in turn, drives the compressor screws through additional gears of the gearing 23.
Projecting in front of the engine is a first driveline 24 (see FIG. 3) which drives a first pump drive gearbox 24a of first hydraulic pump assembly 24b which provides pressurized hydraulic fluid for driving the tracks 14 (tramming). A second driveline 25 drives second pump drive gearbox 25b of a second hydraulic pump mechanism 25a which provides pressurized fluid for carrying out the drilling functions and driving the cooling system for cooling the diesel engine 26, the compressor oil, and the hydraulic oil.
It will be appreciated that the location of the center of gravity of the load supported by the carrier tracks 14 is defined by the layout of the equipment disposed on the platform.
From the standpoint of the design and cost of the carrier tracks 14, it would be desirable for the center of gravity to be centered above the carrier tracks, i.e., be located as closely as possible to a midpoint between the front and rear axles of each track (as the rig is viewed from the side, as in FIG. 1). In that way, each axle would have to be designed to support only about one-half of the load. If, instead, the center of gravity of the load were closer to one of the axles, that axle would have to support more than one-half of the load. Hence, the tracks would have to be oversized for carrying a greater load which is more costly and may result in rig stability problems when tramming. Also, track life is shortened due to the uneven weight distribution. The severity of those problems is dependent upon the distance by which the center of gravity is offset from the midpoint.
On the other hand, from the standpoint of drilling efficiency, it is preferred that the center of gravity be disposed as close to the mast carrier as possible, in order to maximize the pull-down force acting downwardly on the drill bit during a drilling operation.
Therefore, it will be appreciated that the location of the optimum center of gravity of the load is a design compromise between the above-discussed considerations. However, in current drilling rigs there is little versatility in the selection of the optimum location of the center of gravity when manufacturing the rig. That is, as noted above, the motor 26, the screw compressor 22, the hydraulic pump assemblies 24a, 25b, and the gearboxes therefor, occupy such a large portion of the front-to-rear dimension of the platform that there is little ability to adjust the center of gravity.
It would be desirable, therefore, to provide a way of making the design of a drilling rig more versatile from the standpoint of determining the location of the center of gravity of the load supported by the tracks 14.
The gearboxes 24a, 25a of the first and second pump assemblies 24b, 25b are flooded with a fixed quantity of relatively heavy lubricating gear oil which can lead to power loss and difficult start-ups in cold weather, and periodic servicing is required to replace the oil. Pump drive gear boxes on the drill rigs are high maintenance and cost items. Such shortcomings are not present in connection with the compressor gearbox in which thin compressor lubricating oil is continuously circulated therethrough via conduits 23a, 23b (FIG. 3) for lubricating and cooling the gear box, i.e., a so-called “dry” gearbox.
It would be desirable:                to lubricate and cool the hydraulic pump gearings in a manner which reduces the power losses,        to ease the difficult diesel engine start-ups,        to eliminate the need for replacing the lubricating oil in the pump drive gear boxes,        to eliminate expensive, complicated, high maintenance and cost items,        to simplify the overall power train design and construction, and        to reduce the size of the power unit comprised of the diesel engine, the compressor, and the hydraulic pumps        